The giant Perito Moreno glacier in the Patagonian region of Argentina is notable for a couple of reasons. First of all, it is one of only three in the area that are not currently retreating. Secondly, it actually advances right the way across a lake, Lago Argentino, forming an ice dam that can reach more than 70 metres above the surface. The water backs up behind the wall of ice, raising the level of the southern arm of the lake until the pressure becomes so great that the dam collapses. This process repeats in an unpredictable cycle, every…